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Although numerous papers have detailed how
castration-resistant prostate cancers become refractory to Xtandi enzalutamide and
other second-generation androgen receptor
antagonists, comparatively little is known about how the disease eventually
evades CYP17 inhibitors such as Zytiga abiraterone.
Findings from a Cleveland Clinic team could change
that, as the group has detailed how some tumors engage an alternative synthesis
pathway for androgen.1

Androgen
receptor antagonism and inhibition of CYP17 (cytochrome P450 17 a-hydroxylase/C17, 20 lyase) are the mainstays
of castration-resistant prostate cancer (CRPC) treatment. Medivation Inc. and Astellas Pharma Inc. reported $158
million in U.S. Xtandi sales in the first half of the year, and Johnson & Johnson's Zytiga posted
sales of $739 million in that period.

Prior
studies suggested that CYP17 inhibitors may not completely shut down the
CYP17-dependent pathway of androgen synthesis2 and that low levels
of androgen precursors may be able to progress through the pathway.

In
many tumors this should not produce enough of the androgen dihydrotestosterone (DHT) to stimulate tumor growth, but
tumor growth nevertheless occurs.

In
a paper published in Cell, Nima Sharifi and colleagues have solved the
mystery. The team found a mutation in the hydroxysteroid 3b
dehydrogenase 1 (HSD3B1) gene encoding 3bHSD1 that catalyzes a rate-limiting step
in the downstream synthesis of DHT in some CRPCs.

The
mutation may be a mechanism of Zytiga drug resistance, and inhibiting its
enzymatic activity could help treat CRPCs that do not respond to other
therapies.

The
team first studied 3bHSD1 activity in different CRPC cell lines.
Whereas one cell line converted 90% of dehydroepiandrosterone (DHEA) to androstenedione in the DHT
synthesis pathway in
48 hours, the other only converted 10%. Sequencing HSD3B1 in each cell
line identified an A to C nucleotide substitution that resulted in an
asparagine to threonine amino acid substitution in the cell line with enhanced
flux.

To
determine the source of the mutation, the group compared germline DNA from
patients with DNA from their CRPC tumors. The team found that 3 of 25 patients
who were homozygous wild-type for the gene expressed the mutation in CRPC tumor
DNA. This finding suggests that the mutation can arise de novo in
tumors.

However,
the mutation also occurred in the germline DNA of some patients. In 3 of 11
patients who were heterozygous for the mutation, tumors evolved to express only
the mutant gene. These findings suggest that screening for the HSD3B1
mutation could help identify patients unlikely to respond to available
treatments.

Next,
the team assessed whether the HSD3B1 mutation could be responsible for
resistance to Zytiga. In surgically castrated mice with HSD3B1 wild-type
tumors, two of eight animals receiving the drug developed tumors with the mutation,
whereas no tumors from vehicle-treated mice acquired the mutation.

Finally,
the team found that the mutation increased 3bHSD1's half-life
from 2.1 to 27 hours by causing resistance to ubiquitination and degradation.
These findings suggest that the mutation allows more androgen synthesis to
promote tumor growth by stabilizing 3bHSD1.

Drug
development

Sharifi told SciBX that his team's
next step is to identify a small molecule that inhibits 3bHSD1 activity and
thus cuts off the tumor's androgen supply.

Jeff
Hager, senior director of biology at Seragon Pharmaceuticals Inc., said that the
researchers essentially will be starting from scratch.

"One
problem is that there is no proof-of-concept compound with good drug-like
properties that could selectively inhibit the enzyme," Hager said. "They
need to determine first how druggable this enzyme is, and ideally they should
look for a nonsteroidal small molecule inhibitor with improved bioavailability
over steroidal compounds."

Hager
said that abiraterone could alternatively be used as a starting point for
medicinal chemistry in the steroid scaffold. According to the authors of the
paper, the drug weakly inhibits 3bHSD1. However, Hager
cautioned that "hormonal synthesis is a complex process, and there are a
lot of related enzymes and compensatory feedback. An inhibitor that is not
sufficiently selective could affect other important biological processes such
as aldosterone production and
blood pressure."

But
Douglas Jacoby, head of research at Tokai Pharmaceuticals Inc., worried that "the
mouse model does not accurately recapitulate the consequences of 3bHSD1 inhibition that
may occur in humans. Blocking 3bHSD1 will inhibit not only the catalytic
step converting DHEA to androstenedione but also the conversion of pregnenolone or 17-hydroxyprenenolone to steroid
metabolites such as progesterone, deoxycorticosterone, corticosterone and cortisol."

As
a result, he said, it will be necessary "to strike a balance between 3bHSD1 inhibition and
other unwanted consequences resulting from 3bHSD1 blockade. This
can be determined by examining the toxicology of novel 3bHSD1
inhibitors."

Tokai's
Galeterone, a small molecule
that disrupts androgen receptor signaling through a triple mechanism that
involves antagonizing testosterone binding to the androgen receptor, inhibiting
CYP17 and degrading the androgen receptor, is in Phase II testing for CRPC.

Seragon is developing selective estrogen receptor
degraders including ARN-810, which is in Phase I
testing to treat estrogen receptor-positive
metastatic breast cancer. Aragon Pharmaceuticals Inc.
spun out Seragon prior to Aragon's acquisition by J&J in September.

If the Cleveland
Clinic generates an inhibitor of 3bHSD1,
the molecule would likely find immediate use as part of a combination therapy
for CRPC.

Hager
told SciBX, "A real advantage of this potential new target is that
it adds another layer to the antiandrogen therapeutic options. Each level of
therapy lowers androgen levels, and adding a 3bHSD1 inhibitor could
shut down the androgen supply more completely. One could imagine a combination
of LHRH [luteinizing hormone-releasing hormone] agonists to block
testicular androgen, CYP17 inhibitors to block adrenal androgen and a new 3bHSD1-targeting agent
to block the amplified DHT production that the mutation confers. If the
combination is possible from a tolerability perspective, one could imagine
shutting down the hormone production more completely."

William Olson, SVP of R&D at Progenics Pharmaceuticals Inc.,
added that 3bHSD1 inhibition
might not only treat resistant tumors but also prevent resistance in the first
place by blocking the alternative androgen source before tumors evolve to
depend on it.

Hager added, "There are non-overlapping mechanisms
of acquired drug resistance, and altering the drug cocktail to better target
the mutations and genetics of different tumors can all be complementary,
especially when coupled with predictive biomarkers. These drugs all may have
their roles."

Companionship

Olson said that the number of patients
likely to benefit from 3bbHSD1 inhibition might be small. "Based on currently
available data, the 3bbHSD1 pathway appears to be important in a
minority of patients. The mutational status of 3bbHSD1 could serve as
a biomarker to identify those patients who are likely to benefit from this mode
of therapy," he said.

But
Sharifi said that the data in the paper suggest that about 20% of patients with
CRPC may have either the germline mutation with loss of heterozygosity or a
somatic de novo mutation in the enzyme.

"Our
data probably underrepresent the true frequency of the mutation because some of
the patients we studied were progressing but some were from an autopsy
population. If we are looking specifically at progressing tumors, the frequency
might be higher," he said.

Whatever
the exact number, Hager said that given the size of the CRPC market, "north
of 5%-10% affected is a reasonable opportunity and market size."

Sharifi
said that his team's next step for biomarker development is to link the HSD3B1
biomarker to clinical trials for various types of hormone therapy to see if it
predicts response.

Hager
said, "Such a biomarker test could have broad importance because it could
help select patients who produce high DHT levels. Whether these patients are
specifically resistant to abiraterone or not, this indicates that the tumors
likely still depend on androgens, which is valuable information to help guide
treatment decisions. The potential to develop a biomarker test is important
because it suggests an almost immediate application for the findings."

Sharifi
told SciBX that the Cleveland Clinic has filed two provisional patent
applications covering the work that are available for licensing.

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